1. Field of the Invention
The present invention relates to an active power filter. More particularly, the present invention relates to a parallel connection of an active power filter and a load for providing a selectable compensation from a fundamental reactive current, an attenuation of a compound of the harmonic currents of specific orders, and a combination of both of them, with the load being particularly a nonlinear load and with an attenuating ratio of the attenuation being adjustable.
2. Description of the Related Art
Recently, the characteristics of power electronic devices have improved significantly. The power electronic devices with characteristics of high-voltage rating, high-current rating, and high switching speed have been developed due to the improvement of semiconductor manufacturing technique. Power electronic devices are widely applied in electric power equipment, such as an uninterruptible power supply, a motor driver, an arc furnace, a trolley car, a battery charger, a lighting appliance, etc. The electric power equipment may generate a large amount of harmonic currents due to the nonlinear input characteristic of such loads. The harmonic current will pollute the power system and result in serious problems such as transformer overheat, rotary machine vibration, degrading voltage quality, electric power components destruction, medical facilities malfunction, etc. In order to improve the problems of harmonic pollution effectively, many harmonic control standards, such as IEEE519-1992, IEC1000-3-5, IEC1000-3-4, etc., have been established by international research centers.
Traditionally, a passive power filter configured by the inductor and capacitor was used to solve the problems caused by the harmonic pollution in a power system. However, the passive power filter may cause resonance and neighbor harmonic current injection problems that may damage the passive power filter. Additionally, it is quite hard to obtain better filtering performance of the passive power filter since the filter characteristic of the passive power filter may be affected by the system impedance.
Because of the mentioned drawbacks of the passive power filter, active power filters were developed for harmonic suppression recently, which have a conventional structure as shown in FIG. 1. A power source 91 provides a load 92 with electrical power, and an active power filter 93 connects with the load 92 in parallel for providing a compensating current. The compensating current is equal to the fundamental reactive current and the harmonic current of the load. Subsequently, the compensating current is injected into a power line between the power source 91 and the load 92 to suppress the harmonic current of the load 92 and improve the power factor simultaneously.
The conventional active power filters disclosed in U.S. Pat. Nos. 5,677,832 and 5,614,770 with a control structure are shown in FIG. 2. A calculating circuit 80 detects a load current, a source voltage and a DC voltage of the active power filter for calculating a reference signal of compensating current. An output current of the active power filter is detected and subtracted from the reference signal of compensating current by a subtracter 81, and the subtracting result is transmitted to a current controller 82 to generate a control signal. Although the conventional active power filter is able to suppress the harmonic current and improve the power factor, there are still some disadvantages as follows:
1. a conversion ratio of a current sensor for the load current and that for the output current of the active power filter should be pre-known. Otherwise, the compensation performance of the active power filter will be degraded due to the mismatch of the compensating current of the active power filter and the harmonic current of the load; and
2. a generated compensating current includes the harmonic currents composed of all orders and the fundamental reactive current, which results in an application limitation in some applications.
A direct source current compensation method disclosed in “Simplified control method for the single phase active power filter,” IEE Proc. Electrical Power Applications, vol.143, 1996, pp. 219-224 was proposed to solve the first disadvantage above. Nevertheless, its compensating current still includes the harmonic currents composed of all orders and the fundamental reactive current.
Although an active power filter disclosed in U.S. Pat. No. 5,977,660 senses the source current to calculate a reference signal of a compensating current thereby, the generated compensating current still suppresses the harmonic currents of all orders and compensates the fundamental reactive current simultaneously. In addition, microprocessors are generally used to implement the controllers of conventional active power filters. Since the fundamental component dominates the source current after compensating by the active power filter, the harmonic components are too small to be detected from the source current. Especially due to the bits limitations in A/D converters of microprocessor, an evident error will appear in the calculation of reference signal of compensating current for the active power filter. An inaccurate compensating current resulting from the above reasons will degrade the filtering performance of the active power filter.
The present invention provides an active power filter without those disadvantages of the conventional ones. The control method of the active power filter detects a source current, a source voltage and an energy storage capacitor voltage of the active power filter to calculate a reference signal of compensating current. The reference signal of compensating current can be set manually to select as a fundamental reactive current, an attenuation of a compound of the harmonic currents of specific orders, and a combination of both of them. Moreover, an attenuating ratio of harmonic currents of each specified orders is adjustable individually. For improving the accuracy in the calculation of reference signal of compensating current, the detected source current is separated into two parts before transmitting to a microprocessor for calculating a reference signal of compensating current. One is the source current itself, and the other part is the combination of all harmonic components. Furthermore, the control method also operates the active power filter as a parallel operation of a virtual resister and a virtual capacitor at the fundamental frequency to compensate for the fundamental reactive power of the load and the power loss of the active power filter.